Material defects, such as dangling bonds, can reside on the surface or within silicon material. These defects can adversely affect operation of a silicon device, such as a transistor or solar cell, because electron-hole pairs recombine with the defects and are essentially lost. Accordingly, silicon surfaces and bodies are often passivated in order to tie up the defects. In this document, "passivation" means the process of tying up, eliminating, or otherwise rendering inoperative as to current flow, defects in a material.
Several different indicia are used in the industry for indicating the degree to which a surface or body is characterized by defects. "Interface state density," or "D.sub.it," is sometimes utilized by those skilled in the art and is essentially a measure of the number of defects per unit of area. Another variable used in the industry is the "recombination velocity," or "S,", which is a measure of the rate at which electron-hole pairs migrate toward the defects. The recombination velocity S is mathematically proportional to the interface state density D.sub.it. In other words, when the recombination velocity S is low, then the interface state density D.sub.it is low, and vice versa. Furthermore, the lower the interface state density D.sub.it and/or the recombination velocity S of a material, the less defects in the material.
In order to passivate a silicon surface to achieve a low interface state density and/or a low recombination velocity, an oxide layer of SiO.sub.x (x=any number) is sometimes formed over the silicon surface. In a sense, the oxide ties up the dangling bonds of the defects. Typically, a very thin (approximately 100 .ANG.) SiO.sub.x layer is used in high-efficiency silicon solar cells, metal-oxide semiconductor field-effect transistors (MOSFET), and advanced bipolar devices. In regard to solar cells, see M. A. Green, High Efficiency Silicon Solar Cells, Trans. Tech. Aedermannsdorf, 1987, and in regard to bipolar devices, see J. Ahn, et al., IEEE Electron Device Lett., EDL-13, 186 (1992). However, the techniques in the industry for forming the SiO.sub.x layer on the silicon surface generally require undesirable high temperature processing steps. The thin SiO.sub.x layer is typically formed by oxidation at temperatures greater than 700.degree. C. High temperature process steps can limit the degree of miniaturization and can also degrade the quality of starting material.
The deposition of oxides on silicon at low temperature is extremely desirable for achieving greater flexibility in a process sequence, tight dimensional control, and preservation of minority carrier lifetime. Low temperature processing becomes increasingly important as the limits of integration or number of devices on an integrated circuit (IC) chip is increased. Low temperature processing is also crucial for discrete devices, for instance, solar cells as lower quality silicon materials are used to reduce cost of photovoltaic devices. Moreover, the use of a low temperature deposition oxide, such as SiO.sub.x, would be desirable in order to maintain device dimensions within tight tolerance and reduce the process-induced degradation of bulk lifetime. Unfortunately, low-temperature processing generally produces low-quality oxides with high interface state density D.sub.it and high recombination velocity S. In this regard, see E. H. Nicollian et al., MOS (Metal Oxide Semiconductor) Physics and Technology, Wiley, N.Y., 1982.
Thus, there is currently much ongoing research in the industry in developing techniques for producing high quality oxides at low temperature. Many of these techniques utilize plasma-enhanced chemical vapor deposition (PECVD) to deposit oxides. Remote PECVD and plasma oxidation has recently produced a high quality SiO.sub.x /Si interface with a interface state density D.sub.it of approximately 1.7.times.10.sup.10 cm.sup.-2 eV.sup.-1, but the direct PECVD process gives D.sub.it values of approximately 5.times.10.sup.10 cm.sup.-2 eV.sup.-1. These D.sub.it values are slightly higher, but nearly comparable, to thermally grown oxides at high temperatures.
Table A, which is set forth hereafter, summarizes some of the recent developments relative to PECVD oxides, along with the oxide interface formation techniques used to obtain low D.sub.it at low temperatures. In most prior art techniques using low temperature PECVD to deposit oxides, attention has been focused on the interface formation technique during the oxide growth, rather than on the post-deposition treatments.
TABLE A __________________________________________________________________________ T.sub.dep Post-Anneal D.sub.it Ref/Year Interface Formation Technique (.degree.C.) Condition (cm.sup.-2 eV.sup.-1) __________________________________________________________________________ 1/1988 Remote PECVD in situ hydrogen 300 400.degree. C. in N.sub.2, 3.7 .times. 10.sup.10 plasma treatment 30 min 2/1991 Low rate plasma oxidation 350 2.2 .times. 10.sup.10 3/1992 Remote PECVD, wet HF etching, 400.degree. C. in 10, Torr. 5 1.7 .times. 10.sup.10 plasma oxidation 400.degree. C. in N.sub.2, 30 min. 4/1986 Direct PECVD, 350 400.degree. C. in forming gas, 4 .times. 10.sup.10 slow rate (60 .ANG./min) 30 min 5/1991 Direct PECVD, two temperature, 300-350 300.degree. C. in forming gas, 5.3 .times. 10.sup.10 (p) in situ H plasma treatment 60 min 4.0 .times. 10.sup.10 (n) __________________________________________________________________________
It is also important to recognize that there is a fundamental difference between deposited oxides and thermally-grown oxides. In thermally-grown oxides, the SiO.sub.x /Si interface is formed at the end of the process and is located underneath the native oxide. In the case of deposited oxides, the deposition takes place on top of the native oxide, and the native oxide stays at the interface. Consequently, the interface property of the deposited oxides depends strongly on the native oxide. Therefore, the interface formation technique is more critical for obtaining low interface state density D.sub.it in the PECVD oxides.
As indicated in Table A, in a known remote PECVD technique, excited species from a remote oxygen plasma interact with silane (SiH.sub.4) in the deposition zone to avoid ion bombardment damage on the surface. In addition, the use of in situ hydrogen plasma treatment to reconstruct the silicon surface just prior to the deposition was also believed to be a key factor in obtaining low interface state density D.sub.it of about 3.7.times.10.sup.10 cm.sup.-2 eV.sup.1.
Low growth rate plasma oxidation of silicon in dilute oxygen/helium plasma is another promising technique known in the art, as is indicated in Table A. In this technique, a plasma growth oxide forms a high quality interface below the original native oxide, and then the conventional PECVD oxide is deposited on top of the native oxide. Thus, the interface is dominated by a good quality plasma grown oxide rather than the native oxide or the PECVD oxide. A low interface state density D.sub.it value of about 2.2.times.10.sup.10 eV.sup.-1 cm.sup.-2 was reported using this technique in A. A. Bright et al., Appl. Phys. Lett. 58, 619 (1991).
Another known technique utilizes the combination of remote PECVD and plasma oxidation technique, along with wet hydrogen fluoride etching of the native oxide just prior to the deposition. This technique can produce a interface state density D.sub.it as low as about 1.7.times.10.sup.10 eV.sup.-1 cm.sup.-2, which is comparable to oxides prepared by thermal oxidation. In this regard, see T. Yasuda et al., Appl. Phys. Lett. 60, 434 (1992).
Even though the PECVD and plasma oxidation techniques have successfully produced low interface state densities D.sub.it, it still remains difficult to grow such oxides in a commercial PECVD reactor because very low pressure (&lt;10.sup.-8 Torr) is required, as is described in G. G. Fountain et al., J. Appl. Phys. 63, 4744 (1988) and T. Yasuda et al., Appl. Phys. Lett. 60, 434 (1992). Moreover, the growth rate (approximately, 3 .ANG./min) is very slow, as is described in A. A. Bright et al., Appl. Phys. Lett. 58, 619 (1991). On the other hand, direct PECVD is more desirable for commercial scale reactors because PECVD can work at 0.1-1 Torr pressures and the deposition rate is a few hundred angstroms per minute.
In J. Batey et al., J. Appl. Phys. 60, 3136 (1986), it was first reported that low temperature direct PECVD could result in a interface state density D.sub.it in the range of mid 10.sup.10 -10.sup.11 cm.sup.-2 eV.sup.-1 by using relatively low growth rate of 60 .ANG./min, rather than the 500 .ANG./min growth rate in conventional PECVD.
A two-temperature PECVD technique is described in J. S. Herman et al., IEEE Electron Devices Lett. 12, 236 (1991). In this technique, the interface was formed at 300.degree. C. Hydrogen plasma treatment was found to be important for the low interface state density D.sub.it. The D.sub.it value was reported at about 5.times.10.sup.10 cm.sup.-2 eV.sup.-1.
Although the prior art techniques are meritorious to an extent, there is still a significant need in the industry for a low temperature technique for passivating silicon so that the silicon surface exhibits a lower interface state density D.sub.it and a lower surface recombination velocity S than in silicon which is passivated utilizing prior art techniques.